Observationally, there is increasing evidence for spectroscopic and photometric diversity of SNe Ia.
Most SNe Ia can be classified as spectroscopically normal ones, while a small fraction exhibit peculiar properties in some respects [1]}, [2]}, such as the SN 1991T-like overluminous SNe [3]}, [4]}, [5]}, the SN 1991bg-like subluminous SNe [6]}, [7]}, or the SN 2002cx-like subclasses [8]}, [9]}. Based on differences in Si ii velocity evolution, [10]} divided SNe Ia into three subclasses: high velocity gradients (HVG), low velocity gradients (LVG), and FAINT. According to the equivalent width (EW) of Si ii \(\lambda \) 6355 and Si ii \(\lambda \) 5972 absorption lines near maximum brightness, [11]} divided SNe Ia into core normal (CN), broad line (BL), cool (CL), and shallow silicon (SS) subgroups. The sublumious SN 1991bg-like and overluminous SN 1991T-like SNe Ia have large overlap with the CL and SS subclasses, respectively [11]}. Based on the Si ii \(\lambda \) 6355 velocity measured near the time of \(B\) -band maximum, [13]} classified SNe Ia into normal-velocity (NV) and high-velocity (HV) subsets. The HV subclass is found to share some common properties such as red \(B-V\)  color, slow decay in blue bands starting at \(t \approx 40\)  d from the peak, and abundant surrounding CSM [14]}, [15]}, [16]}, [17]}, [18]}. Although asymmetric explosions have been proposed to explain the occurrence of HV and NV subclasses of SNe Ia [19]}, it is difficult to account for the fact that these two subgroups have different birth environments [20]}.
